This invention relates to a foil arrangement which can be used to improve the efficiency, speed, and stability of water-borne craft, both displacement and planing hull types, whether powered by sail or other means. The foil arrangement can be fitted on new as well as existing watercraft.
Although the invention is considered to be of general applicability, the invention will be described with particular reference to sailboards.
A great variety of sailboards are used today. A brief review of the Windsport Magazine Directory board selection chart for 1986 will give some idea of the diversity of size, weight and style of board available. The sailboards listed under "all round recreational" are generally for beginners. At the other end of the spectrum are the "high wind boards" which, while demanding considerable skill from the sailor, provide much greater speed and maneuverability than is possible from the "recreational boards".
The lifting foils, in accordance with the present invention, are designed mainly with high performance boards in mind although, as noted above, the invention can be used in a wide variety of applications.
It is well known that all sailboards sail with a "nose up" attitude. This is due to a combination of factors including hull shape, volume and the hydrodynamic forces acting on the hull. The location of the center of gravity is also of significance. Sailboard hulls typically weight from as little as 12 or 14 pounds to over 40 pounds. This weight is distributed more or less uniformly along the hull length. The mast weighs from as little as four pounds to approximately ten pounds. The sailor obviously contributes the greatest weight and therefore has the greatest effect on the location of the center of gravity.
The sailor, in order to control the sail on the board, must position himself generally toward the rear of the board as illustrated in FIG. 1A. The sailor's position is constantly changing in response to change in wind and/or wave conditions and to the maneuvres the sailor wishes to execute with the board; however, apart from a few, very special exceptions, the sailor's weight is toward the rear. Referring again to FIG. 1A, there is shown a fairly typical representation of a sailboard under way with the sailor positioned on the board for good control of the sail and the board. Under some conditions he could be further aft. With further reference to FIG. 1A, the front of the sailboard is out of the water from a point just rearwardly of the mast. It will also be noted that the wake is curling over the rear deck so that the stern is essentially buried in the wake. This condition causes a substantial amount of drag. This condition, while common, is not always present. FIG. 1B shows a side elevation view of the same board and it will be noted from this that the planing angle of the board is about 8.degree.. This is by no means uncommon for sailboards. The effect of the sailor's weight is indicated with an arrow pointing downward from the center of gravity of his body. The downward component of the hydrodynamic forces on the hull is indicated by an arrow pointing downwardly just forward of the fin.
The high planing angle of a typical sailboard is due mainly to hydrodynamic forces which differ from those normally experienced with a typical planing hull because of the sharply tapered stern characteristic of a typical sailboard. (The sailor's weight, of course, contributes to increasing this angle still further.) The widest point of the sailboard hull is typically close to or even forward of the center of the hull. The pointed stern (pointed as opposed to a wide flat transom), while proven by experience to be the best compromise for best overall performance on a sailboard, nevertheless imposes severe penalties insofar as planing efficiency is concerned.
Reference may be had to the text "Boating in Canada", Practical Piloting and Seamanship, Second Edition, University of Toronto Press, Garth Griffiths, ISBN 0-8020-1817-3 at page 128, where in describing typical planing hulls, it is stated that "the beam of the cross sections does not diminish greatly from amid ship to transom; the width of the planing surface is maintained". A further quote from page 128 of the same text states: "The most effective angle of plane will probably be between 4.5.degree. and 5.5.degree.". Another text entitled "Fluid-Dynamic Drag" by Sighard F. Hoerner, Library of Congress Catalogue Card No. 64-19666, at Chapter 11 page 32, shows the lift/drag ratio of four different shapes of hydro-ski. Among the hydro-skis shown, the flat triangular planform hydro-ski is very close to the stern shape of a sailboard. Examination of the graph provided shows its best lift to drag ratio is at a 5.degree. planing angle.
Using the data from the above two reference books, it can be said:
(1) An inwardly tapering stern on a planing hull tends to be inefficient and increases the planing angle to an undesirable degree which, in turn, increases drag and reduces speed.
(2) The optimum planing angle for a planing hull is between 4.5 and 5.5 degrees.
(3) The optimum planing angle for a sailboard with a stern similar to the hydro-ski discussed above is also about 5.degree..
From the above, and from other observations, the conclusion was drawn that if the planing angle of the sailboard could be reduced to about 4 or 5 degrees, the hydrodynamic drag of the hull would also be reduced which, in turn, would result in greater speed.